Zeolites are crystallised tectosilicates. The structures consist of assemblies of TO.sub.4 tetrahedra forming a three-dimensional framework by the sharing of oxygen atoms. In zeolites of the aluminosilicate type which are the most common, T represents tetravalent silicon as well as trivalent aluminium. The three-dimensional framework mentioned above exhibits cavities and channels which possess molecular dimensions and receive cations which compensate the charge deficit linked with the presence of trivalent aluminium in TO.sub.4 tetrahedra, the said cations being in general exchangeable.
As a general rule, the composition of zeolites may be represented by the empirical formula (M.sub.2/n 0, Y.sub.2 O.sub.3, xZO.sub.2) in their dehydrated and calcined state. In this formula Z and Y denote respectively the tetravalent and trivalent elements of the TO.sub.4 tetrahedra, M denotes an electropositive element of valency n such as an alkali metal or an alkaline earth metal and constitutes the compensation cation and x is a number which may vary from 2 to theoretical infinity, in which case the zeolite is a silica.
Each type of zeolite possesses a distinct microporous structure. The variation of the micropore dimensions and forms from one type to the other leads to changes in the adsorbing properties. Only molecules which have certain dimensions and forms are able to penetrate into pores of a specific zeolite. Owing to these remarkable characteristics, zeolites are particularly adapted to the purification or separation of liquid or gaseous mixtures, for example the separation of hydrocarbons by selective adsorption.
The chemical composition, with, in particular, the nature of the elements present in the TO4 tetrahedra and the nature of the exchangeable compensation cations, is also an important factor involved in the selectivity of adsorption and above all in the catalytic properties of these products. They are used as catalysts or catalyst supports in the cracking, reforming and modification of hydrocarbons as well as in the synthesis of many molecules.